Jul 31, 2014
Light 'needles' thread plasmonic nanoparticles together
Light beams can be used to "weld" a large number of nanoparticles together according to new experiments by researchers at the University of Cambridge in the UK and the Donostia International Physics Center in Spain. The technique could be used to self-assemble sophisticated nanostructures like chiral metamaterials from the bottom up for applications in molecular sensing and to make so-called cloaking devices.
"Lasers are used for welding doors and other car parts in factories and we asked ourselves whether something like this could be done on the nanoscale," says team member Ventsislav Valev. "In a way, what we have done is similar – the light beams that we employed weld together a great number of nanoparticles, and we can monitor the process in real time, again with light. Such a technique could be used to produce nano- and metamaterials in quantities not possible with other bottom-up techniques."
The team, led by Jeremy Baumberg, used femtosecond laser beams as "needles" to "stitch" together gold nanoparticles into long strings. The strings were initially prepared by using barrel-shaped molecules called cucurbit urils to "glue" the gold nanoparticles together. The cucurbit uril molecules act as tiny spacers and separate each nanoparticle from its neighbour by exactly 0.9 nm.
Nanosized hotspots act as conducting "bridges"
"When we shine laser light on the strings of particles in their cucurbit uril scaffolds, this produces plasmons (oscillations of conducting electrons at the surface of a metal that interact strongly with light)," explains Valev. "These oscillating electrons are able to concentrate light energy into tiny nanosized hotspots (hundreds of times smaller than the wavelength of light) that effectively act as conducting 'bridges' between the nanoparticles. Using ultrafast lasers like we did allows us to produce billions of such bridges in a very short space of time, quickly threading the nanoparticles into long strings.
"In terms of applications, our materials could be used to make devices for enhancing molecular sensing (SERS)," Valev told nanotechweb.org. "We are also looking at how to develop our process to create chiral metamaterials through self-assembly in water."
Metamaterials were first made around 10 years ago and are artificial sub-wavelength patterned structures containing arrays of tiny elements such as rods and rings that respond to light and other electromagnetic waves in unusual ways. For example, a metamaterial can be designed to have a negative refractive index so that it bends light in the "wrong" way, or in the opposite direction to normal materials (which normally have a positive index). Such a unique property means that metamaterials have already been used to make "superlenses" that are able to focus light to a point smaller than its wavelength, allowing optical microscopes to view much smaller objects than is possible today. They can also be used to potentially render objects invisible for applications like cloaking devices.
More details of the technique developed by Baumberg’s team can be found in Nature Communications.
About the author
Belle Dumé is contributing editor at nanotechweb.org